What are the laboratory tests for lupus?

Many different laboratory tests are used to detect physical changes or conditions in your body that can occur with lupus. Each test result adds more information to the picture your doctor is forming of your illness.

First, some basic facts about diagnosing lupus:

Lab work alone cannot diagnose lupus.

97 percent of those with lupus will have a positive antinuclear antibody test (ANA).

It’s common for positive lab tests to come and go over time. If this happens, it’s less likely that you will receive a lupus diagnosis, although it remains possible.

It’s very common to get somewhat different results at different labs. However, if a person has active lupus, the ANA will likely be positive at most laboratories most of the time. Other autoantibodies will also be present.

97 percent of those with lupus will have a positive ANA test.

However, it is also helpful to remember that, with lupus, nothing is set in stone. If you continue to have symptoms, seek further medical help to find out what may be causing your health issues.

Routine Blood Tests

Usually, your doctor will first request a complete blood count (CBC). Your blood is made up of red blood cells (RBCs), white blood cells (WBCs), platelets and serum. The complete blood count measures the levels of each. In cases of lupus, these blood tests may reveal low numbers.

Red blood cells carry oxygen to all parts of the body.

White blood cells (lymphocytes and others) help the immune system protect the body against foreign invaders. The white blood cells called T cells and macrophages are directly involved in this protection. Other white blood cells called B cells are indirectly involved by producing antibodies to the foreign substances.

Platelets form in bone marrow; they go to the site of a wound to begin the blood-clotting process.

Blood serum is the fluid portion of whole blood from which certain substances in the clotting of blood have been removed.

Antibody Blood Tests

The body uses antibodies to attack and neutralize foreign substances, such as bacteria and viruses. The antibodies your body makes against its own normal cells and tissues play a large role in lupus. Many of these antibodies are found in a panel or group of tests that are ordered at the same time. The test you will hear about most is called the antinuclear antibodies test, referred to as the ANA test.

Antinuclear antibodies connect or bind to the nucleus or command center of the cell. This process damages and can destroy the cells. While the antinuclear antibody is not a specific test for lupus, it is sensitive and does detect the antibodies that are present in 97 percent of people with the disease.

The ANA can be positive in people with other illnesses or positive in people with no illness. Test results can also fluctuate in the same person. However, lupus is usually the diagnosis when these antinuclear antibodies are found in your blood.

In addition to the ANA, doctors trying to diagnose lupus often look for the following specific antibodies:

Antibodies to double-stranded DNA (anti-dsDNA)

These antibodies attack the DNA, the genetic material–inside the cell nucleus. Anti-dsDNA antibodies are found in half of the people with lupus, but lupus can still be present even if these antibodies are not detected.

Antibodies to histone

Histone is a protein that surrounds the DNA molecule. These antibodies are sometimes present in people with lupus, but are more often seen in people with drug-induced lupus. This form of lupus is caused by certain medications, and usually goes away after the medication is stopped.

Antibodies to phospholipids (aPLs)

These antibodies can cause narrowing of blood vessels, leading to blood clots in the legs or lungs, stroke, heart attack or miscarriage. The most commonly measured aPLs are lupus anticoagulant, anticardiolipin antibody, and anti-beta2 glycoprotein I. Nearly 30 percent of people with lupus will test positive for antiphospholipid antibodies. Phospholipids found in lupus are also found in syphilis, and the blood test cannot always tell the difference between the two diseases. A positive result to a syphilis test does not mean that you have or have ever had syphilis. Approximately 20 percent of those with lupus will have a false-positive syphilis test result.

Antibodies to Ro/SS-A and La/SS-B (Ro and La are the names of proteins in the cell nucleus)

These antibodies are often found in people with Sjögren’s syndrome. Anti-Ro antibodies in particular will be found in people with a form of cutaneous (skin) lupus that causes a rash that is very sun-sensitive. It is especially important for your doctor to look for the Ro and La antibodies if you are pregnant, as both autoantibodies can cross the placenta and cause neonatal lupus in the infant.

Neonatal lupus is rare and not usually dangerous, but it can be serious in some cases.

Antibodies to Sm

These antibodies target Sm proteins in the cell nucleus. Found in 30-40 percent of people with lupus, the presence of this antibody almost always means that you have lupus.

Antibodies to RNP

These antibodies target ribonucleoproteins, which help control chemical activities of the cells. Anti-RNPs are present in many autoimmune conditions and will be at very high levels in people whose symptoms combine features of several diseases, including lupus.

Other Blood Tests

Some blood tests measure levels of proteins that are not antibodies. The levels of these proteins can alert your doctor that there is inflammation somewhere in your body.

Complement

The name of a group of proteins that protect the body against infections. They work by strengthening the body’s immune reactions. Complement proteins are used up by the inflammation caused by lupus, which is why people with inflammation due to active lupus often have low complement levels. There are nine protein groups of complement, so complement is identified by the letter C and the numbers 1 through 9. The most common complement tests for lupus are CH50, C3 and C4. CH50 measures the overall function of complement in the blood. Low levels of C3 or C4 may indicate active lupus. A new combination blood test is using a subset of the C4, called C4d, to help physicians “rule in” lupus and “rule out” other diseases and conditions.

C-reactive protein (CRP)

A protein produced by the liver, and high levels of CRP in your blood may mean you have inflammation due to lupus.

Erythrocyte sedimentation rate (ESR or "sed" rate)

Another test for inflammation. It measures the amount of a protein that makes the red blood cells clump together. The sed rate is usually high in people with active lupus, but can also be high due to other reasons such as an infection.

Blood Clotting Time Tests

The rate at which your blood begins to clot is important. If it clots too easily, a blood clot (thrombus) could break free and travel through the body. Blood clots can cause damage such as a stroke or miscarriage. If your blood does not clot quickly enough, you could be at risk for excessive bleeding if you are injured.

Prothrombin time (PT)

Measures blood clotting and can show whether you may be at risk for not clotting quickly enough at the site of a wound.

Urine Tests

Urine tests are very important because lupus can attack the kidneys -- often without warning signs. The kidneys process your body’s waste materials. Testing a sample of urine (called a “spot urine” test) can reveal problems with the way your kidneys are functioning.

Lupus can attack the kidneys without any warning signs so urine tests are very important.

The most common urine tests look for cell casts (bits of cells that normally would be removed when your blood is filtered through your kidneys) and proteinuria (protein being spilled into your body because your kidneys are not filtering the waste properly). A collection of your urine over a 24-hour period can also give important information.

Tissue Biopsies

A biopsy procedure involves removal of a small bit of tissue that the doctor then examines under a microscope. Almost any tissue can be biopsied.

The skin and kidney are the most common sites biopsied in someone who may have lupus.

The results of the biopsy can show the amount of inflammation and any damage being done to the tissue.

Further tests on the tissue sample can detect autoimmune antibodies and determine whether lupus or another factor such as infection or medication is responsible.